1. Field of the Invention
The present invention relates to an optical waveguide type grating element in which a grating is formed over a predetermined range of an optical waveguide, a production method thereof, a multiplexer/demultiplexer module incorporating the optical waveguide type grating element, and an optical transmission system incorporating the multiplexer/demultiplexer module.
2. Related Background Art
An optical waveguide type grating element is an optical device in which a refractive index modulation (grating) is formed over a predetermined range along a light guiding direction in an optical waveguide (e.g., an optical fiber). The optical waveguide type grating element selectively reflects a signal channel in a predetermined reflection band out of a plurality of signal channels propagating in the optical waveguide, by means of the grating. A multiplexer/demultiplexer module incorporating this optical waveguide type grating element is configured to selectively reflect a signal channel in the reflection band by means of the optical waveguide type grating element to effect multiplexing or demultiplexing of the signal channel. For this reason, the multiplexer/demultiplexer module is applied to WDM (Wavelength Division Multiplexing) transmission systems and others for transmitting signal light (multiplexed signal light) in which a plurality of signal channels are multiplexed.
In general, the optical waveguide type grating element is provided with the grating obtained by forming a refractive index modulation of a definite period Λ along the light guiding direction over the predetermined range of the optical waveguide. This grating selectively reflects light of a wavelength λ satisfying the Bragg condition represented by the equation of λ=2N·Λ, but transmits the other wavelength components. In this equation, N indicates an average effective index of refraction in the index modulated region of the optical waveguide.
When M gratings being index modulations of mutually different periods Λm are formed in mutually different regions along the light guiding direction of the optical waveguide, the optical waveguide type grating element becomes able to selectively reflect M optical components of respective wavelengths λm (=2N·Λm) (m=1−M, where M≧2). However, the optical waveguide type grating element configured to selectively reflect the plurality of wavelength components as described above becomes long in total and high in cost, because the gratings are formed in their respective regions different from each other along the light guiding direction of the optical waveguide.
In contrast to it, there are known optical waveguide type grating elements in which a grating being an index modulation is formed over a predetermined range of the optical waveguide and in which the grating selectively reflects some of signal channels out of the signal channels propagating in the optical waveguide. For example, such grating elements include optical waveguide type grating elements in which an amplitude profile of the index modulation in the predetermined range is given by a sine function, and optical waveguide type grating elements in which index modulations of periods Λm (m=1−M) are superposed in the predetermined range. These grating elements are short in total and low in cost, because the grating being the index modulation is formed in only one range along the light guiding direction in the optical waveguide.